How do we view the Kuiper Belt?

Dr Scott Thomas, University of Cambridge

In August Graihagh Jackson did a whole hour on everyoneís favourite planet-not-a-planet, Pluto and as part of that show she wanted to look into the cloud of dust that Pluto is part of - the Kuiper Belt. Scott Thomas was kind enough to show her around the skies at the Institute of Astronomy.

Scott - The real claim to fame for this telescope was that it nearly discovered the planet Neptune.

Graihagh - Nearly?

Scott - Yes, nearly. So, the story goes that John Couch Adams who was a famous astronomer who worked here at the observatory. He was an undergraduate at Cambridge. At that time, the planet Uranus had been known and there were irregularities in its orbits. So, it wasnít moving in a way that people thought it should. The obvious explanation for this was that there was something out past this orbit that was speeding it up and slowing it down. And John Couch Adams got wind of this idea. He spent his entire summer holidays just trying to calculate the position of this planet.

Graihagh - And they lost.

Scott - Yes. Unfortunately, the French, they took their calculations, they opened up the telescope and I think within an hour, theyíd found it. They were both lucky and they had slightly better maps, I think.

Graihagh - So to me, it kind of looks like scaffolding where itís some elaborate scaffolding on a slant. Where is the actual telescope?

Scott - The actual telescope the bit right in middle there and you can see the eyepiece down the bottom and then at the top, we have a very sophisticated lens cap, which at the moment also includes a plastic bag to keep it dry.

Graihagh - So, you say the lens is at the bottom. So does that mean you lie on the floor to look through it.

Scott - Yes, you do. So, there's a big observerís chair here which is a sort of wooden structure that extends out to the side of the room and it rolls around.

Graihagh - Do you get a duvet as well, and a pillow?

Scott - I've often wondered about this because I can imagine in the middle of winter, it must be freezing and the middle of winter is the best time to observe. So, I imagine you'd wear some pretty heavy duty clothing. I think you should go and sit down. Iíll open up the slit.

Graihagh - Wow! So, the whole roof is moving and with it, the slit is moving around to meet my every need.

Scott - You adjust the focus just by pulling this bit in and out. It really is very difficult to stress just how much easier it is doing this on a modern telescope. Iíll be honest with you. I have no idea what I'm looking at at the moment.

Graihagh - Maybe I'm just blind. So other than the lack of star shots and obviously a bit of luck, these objects are extremely difficult to see. So, what about things beyond Neptune and letís say, Uranus, how are you supposed to see them?

Scott - With a telescope this big, itís very difficult and in fact, what a lot of people donít realise is just how small these things are and how dim they are. So, to spot these things, you need a very large telescope, you need a very high resolution camera. The tricky bit actually can often be Ė you need a long what we call an integration time. So, doing astronomy is kind of like trying to catch raindrops in a bucket where the raindrops are the photons, the raindrops are the light. You can make your bucket bigger which is using a bigger telescope or you can leave the bucket outside for longer, and thatís the integration time. Thatís the time that you look at something. So, this means that while we can get really gorgeous pictures of things like other galaxies that we know stay very still in the sky, an object thatís moving or an object thatís faint that weíre not really sure what weíre looking for can be a lot harder.

Graihagh - The stuff beyond Pluto then, what is it and if it so dark and doesnít reflect much light, and itís really hard to see, how did we discover it in the first place?

Scott - Exactly. So, the Kuiper belt is the edge of the Solar System version of that, just like the asteroid belt is made up of all these chunks of rock that never really made it to form a planet. The Kuiper belt is the stuff beyond Neptune that never really made it to form a planet. So, as for the question of I guess, how do we know whether the stuff is out there, dwarf planets like Pluto, these planets, we can see. Unfortunately, you can only see them with a bigger telescope than this one obviously which is why a lot of them were discovered until relatively recently. Smaller objects are trickier but we do have some information there because even if they're very small and very faint, one of the really interesting things about Kuiper belt is that sometimes things fall inwards from it.

Graihagh - Fall inwards?

Scott - I'm sure you'll be aware that there are some very famous comets, for example, Haleyís comet. Comets like this that have Ė they're called short period comets Ė they're thought to come from the Kuiper belt.

Graihagh - How did it suddenly go from an object to becoming a comet?

Scott - One of the really interesting things about all the stuff out there beyond Pluto is that because it was formed when the Solar System was coalescing and was in its infancy, it never necessarily had the chance to settle down into stable orbits. So, a lot of the stuff that happens out there can be quite chaotic. Perhaps you get a big object passing through that perturbs these things and sends them swinging in towards the centre of the Solar System. Perhaps something happened early in the Solar Systemís formation that sent these things out on very long elliptical orbits and perhaps this is why we see them coming past every so often.

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